The present invention relates generally to manipulating airflow for use as a reactant in a fuel cell system, and more particularly to integrating air supply system components in such a way as to reduce the size, weight and complexity associated with such airflow manipulation.
While conventional power source devices (such as internal combustion engines, including piston and gas turbine-based platforms) are well-known as ways to produce, among other things, motive, heat and electric power, recent concerns about the effects they and their fuel sources have on the environment have led to the development of alternative means of producing such power. The interest in fuel cells is in response to these and other concerns. One form of fuel cell, called the proton exchange membrane (PEM) fuel cell, has shown particular promise for vehicular and related mobile applications, using hydrogen and oxygen as the primary reactants to produce electricity with water vapor as a non-polluting reaction byproduct. A typical PEM construction includes a porous anode and cathode with a solid polymer electrolyte membrane spaced between them. Catalysts, typically in the form of a noble metal such as platinum, are placed at the anode and cathode. In PEM fuel cells, hydrogen or a hydrogen-rich gas is supplied to the anode, where a catalytic reaction between it and the platinum or related catalyst generates protons that can travel through the electrolyte to react with the ionized oxygen at the cathode. The ionization process produces electric current that can supply, among other things, a motor or related motive power device, while the hydrogen and oxygen reaction results in the formation of water at the cathode.
Balanced water levels are required in the PEM fuel cell to ensure proper operation. For example, it is important to avoid having too much water in the fuel cell, which can result in the blockage of the porous anode and cathode, thereby preventing the flow of reactants. Contrarily, too little hydration limits electrical conductivity of the membrane. Exacerbating the difficulty in maintaining a balance in water level is that there are numerous conflicting reactions taking place in a fuel cell that are simultaneously increasing and decreasing its hydration levels. In addition to the formation of water or water vapor at the cathode as discussed above, water can be dragged from the anode and into the cathode by the ionized protons (i.e., hydrogen ions) moving from the anode. This phenomenon, known as electro-osmotic drag, significantly contributes to the removal of water molecules from the anode. Other mechanisms may also be prevalent, including diffusion of water molecules from the cathode to the anode across the membrane, the circulation of hydrogen adjacent the anode to function as an additional water removal source and the removal of water from an oxygen-depleted portion of the cathode. Furthermore, many of these effects are localized such that even if global levels of hydration are maintained in the fuel cell, there is no guarantee that local water balance is maintained. In addition, since typical PEM fuel cells operate at temperatures that are conducive for the evaporation of the resident water and a subsequent drying out of the solid polymer electrolyte membrane, it would be desirable to maintain control over PEM fuel cell temperatures as a way to avoid membrane dehydration.
The manipulation of numerous air properties, including pressure, temperature, relative humidity and mass flow rates, can be used to maintain the desired hydration levels in the fuel cell. One potential method of ensuring adequate levels of hydration throughout the fuel cell includes humidifying one or both of the reactants before they enter the fuel cell. For example, the water produced at the cathode can be used, with appropriate humidification devices, to reduce the likelihood of anode or membrane dehydration. External sources of humidity control (either addition or removal, depending on the need) may also be used. Other approaches employing a combination of the above-mentioned multiple factors (for example, the simultaneous achievement of humidity, pressure, mass flow and temperature levels) to achieve appropriate air conditions may also be used. Of course, the use of humidification, temperature, pressure and mass flow control devices necessitates additional fuel cell system weight, size and complexity (in, for example, the form of pumps coupled to intricate valve networks tied together with precision control mechanisms), as well as reductions in fuel cell output or efficiency in situations where such componentry requires a source of power to operate. Such disadvantages are especially troublesome for vehicle-based fuel cell applications, as the often redundant componentry would take up precious vehicle space otherwise used for passenger, comfort or safety features.
Accordingly, there exists a need for a PEM fuel cell system design and mode of operation that facilitate manipulation of one or more airflow properties without having to resort to approaches that require significant increases in system weight, redundancy, volume or complexity.